The invention relates to an end terminal assembly for sealing small diameter alkaline electrochemical cells. The invention relates to insulating sealing disks which are used to seal the open end of the alkaline cell casing.
Conventional alkaline electrochemical cells are formed of a is cylindrical casing (housing) having an open end. The casing is initially formed with an enlarged open end. After the cell contents are supplied, the cell is closed by crimping the casing edge over an edge of the end cap assembly and radially compressing the housing around the assembly to provide a tight seal. The end cap assembly comprises an exposed terminal end cap plate and typically a plastic insulating member in the form of a plastic sealing disk which forms a plug at the open end of the housing and insulates the terminal end cap plate from the cell housing. A problem associated with design of alkaline cells is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally around the point of complete exhaustion of the cell""s useful capacity. Alkaline cell are conventionally provided with a rupturable diaphragm or membrane within the end cap assembly, for example, as described in U.S. Pat. No. 3,617,386. Such membranes are typically integrally embedded as a thinned out region within the sealing disk are designed to rupture when gas pressure within the cell exceeds a predetermined level. The rupturable vent membrane may be integrally formed as part of the plastic insulating member included within the end cap assembly. Such vent membrane may typically be of circular shape as shown in U.S. Pat. No. 4,537,841. As shown in this latter reference the rupturable membrane can be integrally formed as a thin portion of the plastic insulating member. The rupturable membrane may also take the form of a grooved or circumferential configuration as disclosed in U.S. Pat. No. 5,080,985. The end cap assembly can also be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured.
There is typically an annular cavity or void space carved out of the sealing disk immediately over the rupturable membrane. Such cavity or void space allows gas to accumulate within the cell and provides a path for gas to escape from the cell. Such annular cavity can typically have a width which comprises a substantial portion of the sealing disk radius, for example, at least about 50% of the sealing disk radius.
In one conventional (stacked) end cap assembly for small diameter alkaline cells the open end of the cylindrical casing is sealed by inserting the sealing disk into the open end and crimping the peripheral edge of the casing over the edge of the sealing disk. A paper or plastic washer can the be inserted in stacked arrangement over the crimped edge of the casing and the end cap is inserted over the paper washer. In such stacked arrangement the washer insulates the end cap (negative) from the casing (positive). The end cap can be welded to an elongated current collector which penetrates through the sealing disk and into the cell""s anode material. A representative stacked end cap assembly for small diameter cells having a paper washer between the end cap and casing edge is shown in U.S. Pat. No. 6,025,000. In U.S. Pat. No. 6,025,000 it will be observed that there is a large annular cavity (79) immediately above rupturable membrane (152) within sealing disk 150. The annular cavity (79) as shown in this reference has a width which comprises at least about 50% of sealing disk radius.
In order to provide a tight seal alkaline cells have end cap assemblies which include a metal support disk inserted in a cavity within the plastic insulating member. The metal support disk may have a convoluted surface as shown in U.S. Pat. Nos. 5,532,081 or 5,080,985 which assures that end cap assembly can withstand high radial compressive forces during crimping of the cell""s housing around the end cap assembly. Such support disk allows high radial forces to be applied during crimping. This results in a tight mechanical seal around the end cap assembly at all times. To provide additional support during crimping the peripheral edge of the terminal end cap may also be located in a cavity within the plastic insulating member as shown in U.S. Pat. No. 5,080,985. Such designs, however, can occupy additional space within the cell and can markedly increase the complexity of fabrication, particularly if applied to very small cells.
As disclosed in U.S. Pat. No. 6,025,090 the end cap assembly for small diameter alkaline cells, e.g. AAAA (Quad A) or AAA size cells can be designed without a metal support disk therein. The end cap assembly shown in U.S. Pat. No. 6,025,090, however, includes a paper washer (130) between the casing edge and end cap (200) at the cell""s open end. It would is desirable to provide the alkaline cell with an end terminal assembly without the paper washer. However, when the paper washer is removed there is the problem of insulating the end cap (200) from the casing. If the end cap (200) is also eliminated from the stacked design presented in U.S. Pat. No. 6,025,000, there would be difficulties encountered during mass production of the small size cells. Specifically, when the cells are intermingled during storage or aligned on a conveyer belt, there is a chance that the head of one cell will interlock into the enlarged annular cavity (79) within the sealing disk of another cell.
Thus, it is desirable to provide a seal for small diameter alkaline cells which eliminates the need for a separate end cap and also eliminates the need for a paper (or plastic) washer between the casing edge and the negative terminal.
It is desirable to provide an end terminal assembly for small diameter alkaline cells wherein the end cap assembly does not include a metal support member and also does not require a paper washer between the casing edge and the negative terminal.